Do you think you might add a feature that lets you price adverts at one rate, then if you get x amount of advertisers, the prices changes dynamically.

This would mean that you would set a default start cost for each zone.

So if a standard advert gets 10 active advertisers, price automatically goes up for next 5 or 10 etc. You could enter a % or Ã‚Â£/$ figure you wish it to go up by to each time active user number is met.